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Impromptu or improvised

Impromptu can be a noun, an adverb, or an adjective. It describes something as not being organized or planned in advance.

The plural form of the noun is impromptus.

Improvised is an adjective and is a synonym of impromptu. It carries the additional definition of being created with what’s around, or being makeshift.

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In music, improvisation or an improvised piece is distinctly different than an impromptu piece. Improvisation is creating the music without sheet music or a plan, just letting it come out. This is common in jazz. Impromptu is more like recreating another piece without seeing the sheet music. It can also be called ‘playing by ear’. A person can hear a song and then recreate it. This is not ‘letting it come out’. A person can try many times to find the right melody and it is still impromptu.

Examples

In February, after several hundred people attended a protest against the construction of new hotels in the district, the soon-to-be hotel was occupied by activists, who turned it into a impromptu office to help those on the verge of being evicted from their homes. [The Guardian]

But on Sunday afternoon, his playing was truly convincing only in the repertoire he’s most associated with: a Liszt Hungarian Rhapsody and the most Romantic of the four Schubert impromptus on the program. [Boston Classical Review]

As a lieutenant in Afghanistan’s Helmand province in 2010, one of his squads was struck by an improvised explosive device. [The Denver Post]